1. Field
The present invention relates to a cellulose acylate film, a protective film for polarizing plate, a polarizing plate using the protective film for polarizing plate, and a liquid crystal display device.
2. Description of the Related Art
Recently, liquid crystal display (LCD) devices, especially those for use in TVs are becoming larger, and hence high definition and low price are demanded for LCD devices more and more. In addition, since LCD devices are expected to be used more frequently for outdoor applications, mainly as electronic sign boards in the future, there is a demand for LCD devices which can withstand more severe environments than in the past.
Meanwhile, it is required to impart some functions such as scratch resistance, anti-reflection and antistatic property to the surface of a viewer side polarizing plate used in LCDs. An ordinary method for enhancing scratch resistance is to provide a layer with high hardness on a protective film of the polarizing plate, such as a layer hardened by irradiating an active energy ray (hereinafter referred to as “an active energy ray-cured layer”). Further, it is known that enhancing the surface hardness of the protective film for polarizing plate itself is also effective. Japanese Patent Application Laid-Open No. 2005-206721 discloses a method for using a cellulose acylate film having a high surface hardness as a protective film for polarizing plate.
A polarizing plate widely used in LCD devices is configured such that a polarizer using polyvinyl alcohol (PVA) and iodine is sandwiched between protective films for polarizing plates such as cellulose acylate film. However, the polarizer using PVA and iodine has a drawback in that the polarizer performance is prone to deteriorate under high temperature and high humid environments, and thus, is needed to be improved in order to satisfy the performance required for outdoor applications.
Outdoor applications require more rigorous durability for an active energy ray-cured layer as compared with indoor applications. Among them, stability against light is an especially important criterion. However, active energy ray-cured layers in the related art have a problem in that they are easily peeled from a cellulose acylate film when irradiated with light for a long period of time, and thus, are needed to be improved.
Generally, a means for enhancing light resistance is to add an ultraviolet ray absorbent or an antioxidant. For example, Japanese Patent Application Laid-Open No. 2009-167416 discloses a resin film to which a triazine-based ultraviolet ray absorbent and a hindered amine-based antioxidant are added. Further, International Publication WO 2007/072643 and Japanese Patent Application Laid-Open No. 2006-123513 disclose a cellulose acylate film containing a hindered amine-based antioxidant.
However, the present inventors have reviewed and found out that the film obtained by the method as described in Patent Japanese Patent Application Laid-Open No. 2005-206721 has insufficient adhesion between a hardcoat layer and a cellulose acylate film when irradiated with light for a long period of time. Meanwhile, the methods of Japanese Patent Application Laid-Open No. 2009-167416, International Publication WO 2007/072643, and Japanese Patent Application Laid-Open No. 2006-123513 may exhibit a certain improving effect on adhesion between a hardcoat layer and a cellulose acylate film when irradiated with light for a long period of time. However, it was found out that there is a problem in that scratch resistance is impaired when a hindered amine-based antioxidant is added in a large amount.
The present invention has been made in consideration of the above situation, and an object of the present invention is to provide a cellulose acylate film which is excellent in scratch resistance, and sufficiently maintains the adhesion with an active energy ray-cured layer such as a hardcoat layer when irradiated with light for a long period of time. Further, another object of the present invention is to provide a polarizing plate and a liquid crystal display device using the protective film for polarizing plate.
Conventionally, an affinity between the surface of the hardcoat layer and the cellulose acylate film and/or a degree of crosslinking in the hardcoat layer has been considered as important factors for adhesion between a hardcoat layer and a cellulose acylate film. However, the present inventors have studied intensively studied, and as a result, found out that peeling between the hardcoat layer and the cellulose acylate film may be sometimes caused by the brittle fracture of the surface layer of the cellulose acylate film. Furthermore, the present inventors have found out that a radical generated by photoreaction of additives in the hardcoat layer or the cellulose acylate film causes depolymerization of the resin (cellulose acylate) contained in the cellulose acylate film and the depolymerization is one large factor for the brittle fracture. The present inventors also have found out that, when an antioxidant (a hindered amine-based compound) is added to the cellulose acylate film in a specific amount, the adhesion between the active energy ray-cured layer such as a hardcoat layer and the cellulose acylate film is improved.
Furthermore, the present inventors have found out that the reduction in free volume of the cellulose acylate film enhances the surface hardness, as well as more effectively improves the adhesion between the active energy ray-cured layer such as a hardcoat layer and the cellulose acylate film by the hindered amine-based compound. It is supposed that this is caused because the hindered amine-based compound is suppressed from being diffused into the active energy ray-cured layer as the free volume of the cellulose acylate film is reduced.